@cleverca22 pointed out:

nix-repl> (import <nixpkgs> { system = "x86_64-darwin"; }).acl.meta
error: Package ‘acl-2.2.52’ in ‘/nix/store/jk5dvrv6w9bcgh88g0x7clk19df3q28f-nixos-17.03pre96925.1c50bdd/nixos/pkgs/development/libraries/acl/default.nix:29’ is not supported on ‘x86_64-darwin’, refusing to evaluate.

This is unfortunate because it would be extremely handy to have a function that, given a package, could tell me if it is compatible with a system (most often, my current system). With such a function we could safely make certain features/dependencies optional without having to manually duplicate platform compatibility information with the likes of isDarwin, isLinux, etc.